The use of products formed of polytetrafluoroethylene (PTFE) in medical applications is well known. Products such as implantable grafts, implantable patches, catheter tubing and the like may be derived from extruded tubing of PTFE.
PTFE tubing is normally manufactured by a paste extrusion process. Screw injection extrusion which is typical of most thermoplastics may not be effectively used with PTFE because PTFE resin does not exhibit sufficient fluidity even when heated. In the paste extrusion process a "green tube" is formed. A green tube is a tube of PTFE that must be subjected to secondary operations before it yields a usable medical product. Such secondary operations may include stretching and expanding the tube under various conditions of time, pressure and temperature. The paste extrusion process tends to produce a tube which has a fibrous state where its fibrils are generally longitudinally aligned in the direction of extrusion. This fibrous state formation is particularly evident where the PTFE paste includes a lubricant to assist in extrusion. Extruded tubes having fibrils longitudinally aligned in this fashion exhibit low radial or hoop strength. Such a tube is highly susceptible to tearing or rupturing.
Attempts have been made to modify the structure of extruded PTFE tubing. Such attempts seek to manufacture extruded PTFE tubing having non-longitudinally aligned fibrils where the fibrous state formation includes fibrils aligned transversely to the extrusion direction. One attempt in the vascular graft art is shown in U.S. Pat. No. 4,743,480. This technique employs a screw tip on the extrusion mold to reorient the fibrils during the paste extrusion process. The pitch of the screw tip tends to twist the fibrils during extrusion.
In the mechanical art area, U.S. Pat. No. 4,225,547 employs co-rotation to manufacture pipes and wire jackets. In this co-rotation example, the mandrel and the outer portion of the extrusion die are counter-rotated with respect to one another. While this tends to orient the fibrils in both the longitudinal and transverse direction, as set forth in the '547 patent a suitable product is only obtained by heating the tube during extrusion. In this process the die is heated to a temperature significantly above the normal paste temperature. Heating the die while counter rotating the die components results in a product where the fibrous-state formation in the direction perpendicular to extrusion is greatly enhanced.
However, the process described in the '547 patent has several disadvantages. First, heating the metal die components so as to maintain the PTFE paste at an elevated temperature results in thermal expansion of the die components. The differing rates of thermal expansion of these discrete die components may result in gaps being formed between such components. Tubular products formed thereby may exhibit deleterious marks or scarring. Further, leaking of PTFE paste into the die apparatus through the gaps formed between components may adversely effect the operation of the components. Second, lubricant which is normally mixed in with the PTFE paste, to increase fluidity and ease of manufacture, has a tendency to evaporate at elevated temperatures resulting in tubing that is harder and more difficult to handle.
It is therefore desirable to provide a paste extrusion process which overcomes many of the above-evidenced disadvantages.